Question

Dan sold half of his comic books and then bought nine more he now has 18 how many did he begin with

Answer 1

i would say because of how manny you would have to add so you would hget 18+9

Answer 2

he bought 9 more so do the opposite.
18 -9 is 9.
he sold half so do the inverse wich is multiply 9 by 2. You will get 18.
Therefore, Dan begins with 18 comic books.